Digital radiography (DR) is increasingly preferred as an alternative to both film-based and Computed Radiography (CR) imaging technologies that use photosensitized film or photostimulable storage phosphors to obtain image content from radiation exposure. With digital radiography, the radiation exposure energy that is captured on radiation sensitive layers is converted, pixel by pixel, to electronic image data which is then stored in memory circuitry for subsequent read-out and display on suitable electronic image display devices. One driving force in the success of digital radiography is the ability to rapidly visualize and communicate stored images via data networks to one or more remote locations for analysis and diagnosis. With DR imaging, this can be done without the delay that results when film is first developed and checked, then either packaged and delivered to a remote location or input to a separate scanner apparatus to provide digitized image data.
Flat panel digital radiographic (DR) imaging systems enjoy a number of advantages over conventional film-based or earlier CR systems. Among its salient advantages is the capability of the DR system to obtain radiographic image data without the need for an operator or technologist to move, handle, process, or scan any type of imaging medium following exposure. Data that is downloaded directly from the DR receiver panel is then quickly available for viewing and diagnosis on-site or at any appropriately networked viewer workstation.
Due to factors such as size, weight and expense, earlier flat panel digital radiographic (DR) imaging detectors were permanently mounted in table and wall bucky structures specially designed to accommodate them. More recently, due to technological advances in solid state electronics that provide reduced size and power requirements, a more portable and retrofittable type of digital detector is envisioned. Ideally, a more portable DR detector would have the data-gathering advantages of earlier detectors, but with reduced weight and size that could allow its conformance to the ISO-4090 35×43 cm standard cassette profile. This would allow the DR detector to be fitted into existing table or wall x-ray units that also conform to this ISO standard. This conformance promises to expand the usability of DR detection as a replacement for existing film and CR cassette-equipped x-ray rooms, obviating the need to upgrade or modify existing x-ray table and wall equipment, as is done currently. As a result, retrofit DR detectors would be usable with systems that are now constrained for use only with film and CR detectors.
In addition to reduced size and weight, it would be desirable to provide a truly portable digital detector that is untethered for wireless communication and that contains on-board battery power. With these additional advantages, the portable DR detector can be more easily used with existing x-ray imaging systems. This would help to provide a detector that can be readily moved from one location to another as needed, without the cumbersome requirements and risks imposed by the need to connect power or data cables.
DR technology offers promise as a possible retrofit to existing imaging systems and may help to improve workflow, efficiency, and timeliness in providing diagnostic information, at reduced upgrade cost. However, a number of problems remain to be successfully addressed. Among these problems are difficulties related to noise from nearby equipment, such as earlier bucky units. Because of their large sensing area and overall sensitivity, DR detectors are particularly susceptible to ElectroMagnetic Interference (EMI) from surrounding electromagnetic sources, such as grid motor drives and automatic exposure control power supplies. Extraneous electromagnetic noise interferes with the quality of the captured X-ray image data and can introduce artifacts that compromise the value of these images in clinical diagnostic applications. Low frequency magnetic fields have been found to be particularly problematic because of the difficulty in shielding against this type of EMI.
To appreciate the problem of shielding for this type of device, it is useful to first consider the component-level structure of the DR detector and the nature of the induced noise. The schematic diagram of FIG. 1 shows representative sensing and data gathering circuitry of the radiological image detector. A radiological imaging detector panel 10 is an array with millions of photosensors arranged in a row-column matrix and row and column readout lines 20 and 22 respectively. For each pixel 14, a photosensor 12, such as a photodiode, produces an electric charge that is proportional to the amount of radiant energy it receives. The charge produced by each sensor is read out using an array of charge amplifiers 26. Each photosensor has a connection to a particular column readout line through an associated thin film transistor or TFT 16. A bank of gate drivers 18 selectively turns on a given row of thin film transistors, allowing charge from the photosensors to flow into each of the charge amplifiers 26. Charge amplifiers 26 then convert the charge to a voltage that is provided on a signal bus 30 and can then be readily converted to a digital value through an Analog-to-Digital A/D converter 28 and through associated multiplexer (MUX) 32 circuitry. Related support circuitry for the pixel array includes a bias supply 34 that provides bias lines 24 to photosensors 12.
For any pixel 14, the amount of charge generated in photosensor 12 during an image readout operation is on the order of tens of picocoulombs. This extremely small signal travels through the long column readout lines 22 that are distributed over the imaging area of the panel. For a typical imaging detector panel 10, readout lines 22 can be up to 43 cm long, providing a path of significant length for induced noise.
The schematic diagram of FIG. 2 shows photosensor readout electronics for each pixel 14 in more detail. Here, individual photosensor 12, shown as a photodiode, is switched by TFT 16, under control of a gate driver 50 along a signal path 52 to charge amplifier 26 through the column readout trace line 22. The inherent resistance and capacitance of circuit traces are represented for both signal path 52 and readout line 22, enclosed in dashed outline as an equivalent circuit 54. With a switch 56 open, charge amplifier 26 integrates the signal and, with a switch 68 closed, provides a reference charge value to a storage capacitor 64 in a Correlated Double Sampling (CDS) switch 60. A switch 66 provides the signal from amplifier 26 to a storage capacitor 62 once the signal representing pixel charge level is obtained.
Interference from magnetic fields arises when a conductor is placed within a changing magnetic field. This is sometimes described as change in flux linkage of the field with a conductive loop. The changing flux in a conductive loop results in an induced electromotive force or voltage. If the conductor is part of a high impedance, low voltage signal path, the magnetically induced voltage adds to the original signal as noise that interferes with the measured signal.
If readout lines 22, of FIGS. 1 and 2 link a changing magnetic flux, there can be an induced error voltage in the readout column line that degrades the image quality of the detector. According to Faraday's law, the magnitude of the induced voltage is given by:
                    ɛ        =                  -                                    ⅆ              ϕ                                      ⅆ              t                                                          Equation        ⁢                                  ⁢        1            where ε is the induced electromagnetic force (emf) in volts and φ is the magnetic flux linking a single turn in webers.
From Equation 1, it is apparent that an induced voltage in the conductor with a time varying magnetic field is directly proportional to the time rate of change of the flux linking the conductor. Both PWM motor drives and flyback transformers, for example, have very high pulsing inductor currents that generate high dφ/dt values. Their frequencies usually fall in the 20 kHz to 100 kHz range, over which a Faraday type shield is not very effective. Since the readout lines, shown in FIGS. 1 and 2, can be up to 43 cm long, they can be quite susceptible to the extraneous magnetic fields that potentially exist in some radiological imaging suites.
Referring again to FIG. 2, to accurately integrate the small detected charge to a voltage level that can be converted to a digital signal, the circuitry of charge amplifier 26 presents a very high impedance to the detected signal. This high-impedance circuitry is sensitive and, at the same time, very susceptible to extraneous electric and magnetic fields that introduce noise into the signal. Once extraneous noise has been introduced, it can be difficult or impossible to remove. Measures taken to keep external electromagnetic noise from the sensitive electronics of the detector typically include shielding.
A common type of shielding that is employed for electronic devices is commonly termed Faraday shielding, in which sensitive high-impedance electronics are enclosed inside a housing that has a conductive material of some kind. The Faraday shield mechanism can be a metal enclosure made of aluminum or a plastic housing onto which a thin conductive coating has been applied. The conductive material of the housing is then connected to the same ground point as the ground of the electronics. This arrangement, using basically the same principles employed with coaxial cable, effectively shields the circuitry from extraneous electric fields.
High frequency magnetic fields at frequencies in excess of 1 MHz can be shielded using Faraday shielding techniques. This is due to the fact that an AC magnetic field induces eddy currents in the conductive metal of the enclosure that oppose the applied magnetic field.
Eddy current cancellation, however, becomes less and less effective as the frequency of the magnetic field decreases below a certain point. For example, magnetic fields in the range of 60 Hz to 100 kHz exhibit very little attenuation from a conductive Faraday shield. A DC magnetic field (0 Hz) will pass completely through a piece of aluminum or copper because no eddy current is formed.
In practice, Faraday shielding has little or no value for magnetic field frequencies below 100 kHz. Thus, frequencies in this lower range, such as frequencies from 60 Hz power lines, remain a potential source of interference for high impedance electrical circuitry, even where Faraday shielding is used.
Unfortunately, there can be any number of sources of low frequency magnetic fields in areas of radiological imaging detector use. Certain types of equipment are known to radiate low frequency electromagnetic fields. Examples readily found in and around X-ray tables and Bucky drawers include PWM motor drives used in grid motor drive units and flyback transformers found in voltage supplies. Both of these sources can generate magnetic fields that fall in the frequency range of 20 to 100 kHz, a range not effectively shielded using conventional Faraday shielding.
By design, components that generate significant levels of EMI are not used for built-in or integrated DR systems. From the beginning stages, such systems are carefully designed so that possible interference from system components is eliminated or at least minimized. However, this is not often the case with earlier x-ray systems that were originally designed for use with film or CR media. As a result, portable DR equipment that is to be used as retrofit for existing hardware is protected, inasmuch as possible, from potential sources of EMI at intermediate- or lower-frequency ranges. This protection accounts both for environments where there are known and predictable sources of EMI and for conditions in which EMI is not easily predicted, wherein the relative location and intensity of the generated EMI field can be unknown or changing.
It is known that an external, lower-frequency magnetic field can be redirected around a circuit and prevented from interfering with it by enclosing sensitive electrical circuitry inside a properly designed enclosure; as shown in FIG. 3. Materials effective for shielding low frequency magnetic fields in this way have certain desirable ferromagnetic properties. Typically, acceptable materials include very soft magnetic materials such as nickel-iron based alloys that exhibit high permeability.
There are a relatively limited number of materials available for low-frequency magnetic shielding, and these materials have some limitations with respect to weight and workability. Permalloy and Mu metal are two examples of shielding materials commonly used for this purpose. These materials are available in a range of different shapes and sizes. Sheet forms typically vary in thickness from about 0.002 to 0.010 inches for foils and up to about 0.065 inches or more for sheets and plates.
Additionally there are also some relatively new magnetic shielding materials now available based on nanocrystalline iron alloys. A nanocrystalline material exhibits an extremely fine-grained microstructure with grain sizes as small as 10 nanometers. Conventional soft magnetic materials such as Permalloy and Mu metal have much larger grain structures that can exceed 1 μm. It has been generally observed that as the size of crystal grain structures decreases, the soft magnetic properties of a material tend to degrade and the coercive force increases. However, it has been found that this relationship actually reverses for grain structures below 100 nanometers.
Nanocrystalline materials with high permeability and large surface area suitable for shielding applications can be manufactured using several different manufacturing techniques. One technique for the fabrication of FINEMET® uses rapid quenching of a molten alloy consisting of Fe, Si B and other trace elements at one million ° C./second. This produces strips of an amorphous metal with extremely small uniform crystals. To achieve large surface area, strips of this material are then welded together with a small overlap between adjacent strips. The welded strips are then laminated between layers of plastic. This material is available in rolls up to fifteen inches wide that can be conveniently cut to the desired length and width and attached to a structure with double sided adhesive tape.
It has also been found that materials having nanocrystalline microstructures can be produced using pulsed electric fields during an electrodeposition process. Normally electrodeposition of a metal produces crystals with random orientations and grain sizes in the micrometer scale. However, modifying this process by pulsing the electric current during the plating process alters the growth condition of the crystals, thereby producing a smaller grain size. Additionally, it has been found that alternately reversing the pulse current, for a short duration, produces even finer grain structures. This is due to the fact that during the time the electric field is reversed, a process of electro-decomposition occurs that creates nanopores in the plated structure. These nanopores are then filed in during the next forward current pulse. This constrains the growth of crystal grains to the nanometer scale.
Nanocrystalline iron-nickel alloys that exhibit high permeability have been produced using pulsed electrodeposition. While iron and nickel in various proportions make up the bulk of this type of material, other elements in smaller proportions can be introduced to enhance the magnetic performance of electrodeposited nanocrystalline materials. For the purpose of the present disclosure, these minor variations are considered within the scope of the term nanocrystalline iron-nickel alloys.
Using an electrodeposition process such as this, a layer of nanocrystalline iron-nickel alloy having high permeability can be produced and can be directly applied to structures of relatively large surface areas to serve as magnetic shields, without the necessity of first cutting the material to shape and then applying an adhesive to the structure.
Soft magnetic materials used for low-frequency magnetic field shielding are readily magnetized and demagnetized. These materials, used primarily to control or channel the flux of the magnetic field, typically have intrinsic coercivity less than about 10 Am−1. A parameter that is often used as a figure of merit for soft magnetic materials is the relative permeability μr which is a measure of how readily the material responds to an applied magnetic field. A material with high permeability has lower magnetic reluctance than a material with low permeability. Magnetic materials of this type provide a low reluctance path for the magnetic field to follow, rather than a higher reluctance path, such as that of air. By way of comparison, air is used as a standard, so that relative permeability of a material is conventionally expressed relative to that of air, at a given frequency. Air has a relative permeability of 1 at a frequency of 1 kHz, while certain Mu or Permalloy metals may exhibit a relative permeability that is from about 5,000 to as much as 250,000 or more, at a frequency of one kilohertz.
Magnetic shielding techniques using highly permeable materials such as Permalloy and Mu metal have been employed for a number of years to shield devices susceptible to interference from low-frequency magnetic fields. Applications of this type for example have been used for the shielding of photomultiplier tubes, CRT or cathode ray tubes and sensitive optical gyroscopes. It has been found that layering of ferromagnetic materials is an effective magnetic shielding technique, with successive shielding layers separated between layers of non-ferromagnetic materials. The non-ferromagnetic material might be any of the non-ferrous metals such as aluminum or brass, various plastics, or air.
One example with layered shielding is shown in the cross-sectional view of a container in FIG. 4. Here, a layer of magnetically permeable material 72 is disposed against inside and outside walls of the container formed from non-ferromagnetic material 74. Each layer of ferromagnetic material contributes to the overall attenuation factor. This dual-layer arrangement provides improved performance over a single layer, even where the single layer may be thicker than the combined total thickness of the two separate layers.
The technique described with reference to FIG. 4 has been used, for example, to shield sensitive optical gyroscopes as described in U.S. Pat. No. 6,627,810. When using such a layered technique, the number of individual layers of magnetic material is not limited to two; there have been applications that use three or four distinct layers, where each ferromagnetic shielding layer is separated from the next with a layer of non-magnetic material. In some applications, the material in each magnetic layer may have different properties, including different permeability characteristics. Use of lower permeability materials on the outside of a structure provides a higher saturation that reduces the strength of the field for the next layer. It can also be advantageous to use different magnetic materials on different portions of the layers to take advantage of other properties such as lower cost or greater durability. With reference to FIG. 4, for example, it might be desirable to use a different material 72 on the inside layer as opposed to the outside.
Although the surface coverage and layering approach described with respect to FIG. 4 is relatively straightforward in concept, there can be considerable difficulty that prevents this approach from being used effectively in practice. For example, the relatively simple geometry of the cylinder described in the '810 disclosure lends itself readily to the application of pre-formed shielding material 72. However, even if a preformed shield is used, its magnetic shielding performance can be seriously degraded if the material is stressed, such as by being bent or folded. Any type of mechanical stress on the low permeability material, such as: bending, forming, shearing, punching, drawing, or subjecting it to high temperatures such as those used during welding, can cause work hardening of the material. In the embodiment of FIG. 4, for example, the internal and external shielding layers of the container have 90-degree bends in order to conform to the structure.
Fringe effects present additional problems for enclosure shielding. In order to shield effectively, the magnetic shielding material encases the entire detector, preferably without gaps between any segments of shielding that would allow fringing. FIG. 5 shows a cross-section of an enclosure with a cover section 40 and a bottom base section 42 and shows how fringing can occur where segments of a shield are discontinuous even with cover section 40 in place. Fringing can be a particular problem for the DR detector because, during the lifetime of the detector, access can be required to internal components as well as to external connectors. Fringing is likely wherever there are gaps or breaks in continuity in magnetic shielding material, such as where there are removable covers, cable access ports, or components provided on the surface of the device.
It is noted that the magnetic shielding shown in FIGS. 4 and 5 cannot be fabricated without performing some type of bending over sharp radii, drawing or welding operation on the high permeable material before assembling it onto a cylindrical structure as shown in FIGS. 4 and 5. Any of the aforementioned stressful operations that work-harden the magnetic material effectively damage or destroy its beneficial shielding properties. Following any operation causing mechanical stress, it is necessary to restore the material's high permeability and thus its magnetic shielding properties by an annealing process.
Annealing for this type of material can be a fairly complex and costly operation. In annealing, the soft permeable alloy, typically Mu metal or Permalloy, is subjected to high heat, either in a vacuum or in a controlled atmosphere, such as in a hydrogen atmosphere. During the heating cycle the magnetic material is raised to a temperature of around 2100 degrees Fahrenheit and held at this temperature for several hours followed by a controlled cooling cycle to maximize the permeability.
Annealing also introduces problems. High annealing temperatures can cause the shielding material to become quite soft, resulting in the loss of dimensional integrity of the prefabricated part. Parts that are to be subjected to high annealing temperatures after fabrication is constructed with sufficient thickness to prevent excessive warping during the heating cycle. Thus, for example, thin foils would not be appropriate for pre-formed structures; a preformed structure formed from a thin foil will readily become curled and warped and be completely useless after annealing. Thus, in order to make it practical to use pre-formed magnetic shield structures, the prefabricated parts have sufficient thickness to preserve dimensional tolerances during the annealing process. For Permalloy and Mu materials, this necessitates material thicknesses far greater than a typical foil thickness of 0.002 to 0.004 inches. This adds bulk and weight to the completed, shielded device and, although this may not be a problem for some types of equipment, added bulk and weight are not compatible with what is needed for shielding a portable DR detector.
For portability and industry acceptance, a portable DR detector design meets fairly stringent dimensional profile and weight requirements. These two factors require that any type of shielding material be as light and thin as possible, essentially precluding the use of any known material other than a relatively thin Mu foil alloy. The fairly complex internal and external shape of a retroffitable detector adds further to the shielding problem and greatly complicates the design problems for prefabricated parts, due to the difficulty of pre-forming a thin soft foil into complex shapes, as discussed above, without applying some type of potentially damaging mechanical stress. Further, even where thin Mu metal foils are provided without being pre-formed and annealed, conventional methods for adhering Mu metal foils to a complex folded structure are highly subject to human error and are likely to result in disappointing manufacturing yields.
Full encasement of the DR detector requires a layer of ferromagnetic material on the top cover of the detector where the X-rays enter. This layer of material will unfortunately absorb some portion of the X-ray energy entering the detector and reduce its overall efficiency, limiting the detective quantum efficiency or DQE. This may potentially require the level of X-ray exposure to be increased, exposing the patient to a higher dose of radiation.
In summary, conventional shielding techniques are not compatible with the design and intended function of the DR detector for a number of reasons, including at least the following:
(i) Undesirable levels of X-ray attenuation. Unfortunately, conventional coatings or coverings that are known to be effective low-frequency EMI shields tend to be formed from materials that attenuate the X-ray signal. Use of such materials in a conventional shielding arrangement would require an increased radiation dose in order to obtain the diagnostic image.
(ii) Excessive weight. Conventional shielding materials themselves can add significant weight to the DR detector, making the device less portable and less desirable as a replacement for its film or CR counterparts.
(iii) Constraints on dimension. In order to fit within the ISO-4090 35×43 cm standard cassette profile and provide sufficient imaging area, shield materials are limited as to thickness.
(iv) Need for full encasement. The DR receiver panel is fully encased within the shield. Gaps between portions of a shield are undesirable due to fringing.
(v) Difficulty in working with materials. This applies to both shaping the shield materials and applying them to the detector surface.
Unable to resolve these difficulties with conventional shielding techniques for effectively shielding the DR detector while meeting stiff dimensional, weight, and performance requirements, researchers have looked elsewhere for ways to counter the EMI problem. As just one example, U.S. Pat. No. 7,091,491 entitled “Method and Means for Reducing Electromagnetic Noise Induced in X-Ray Detectors” to Kautzer et al. states that EMI shielding for such detectors is not feasible and discloses sampling an additional exposure cycle for compensation. However, such techniques assume that the induced noise distribution is at least somewhat constant, which is not the case for many types of EMI.
Thus, a portable DR detector desires EMI shielding that is compatible with requirements for low weight, has minimum impact on dimensions, provides suitable shielding performance, and can be feasibly manufactured.